1. Field of the Invention
The present invention relates to an internal combustion engine fitted with a sensor arranged to detect knocking. The present invention also relates to a straddle-type vehicle equipped with the engine.
2. Description of the Related Art
An internal combustion engine can cause knocking in some cases depending on its operating conditions. Knocking should be avoided as much as possible because it results in, for example, unusual noise and performance degradation of the internal combustion engine. Conventionally, it is known that a sensor to detect knocking, that is, a knock sensor, is fitted to an internal combustion engine. It is also known that, upon detecting knocking by the knock sensor, an action such as changing ignition timing is taken.
JP 2004-301106 A discloses a water-cooled engine in which a knock sensor is fitted to a cylinder block.
Knocking occurs in a combustion chamber. When knocking takes place, the vibration resulting from the knocking propagates from the combustion chamber to the cylinder block, and then to the crankcase. Since the cylinder block is closer to the combustion chamber than to the crankcase, knocking can be detected more accurately when the knock sensor is provided on the cylinder block than when the knock sensor is provided on the crankcase. The cylinder head is provided with an intake valve, an exhaust valve, and a cam mechanism to open and close the intake valve and the exhaust valve. Although the cylinder head is close to the combustion chamber, the cylinder head is more affected by the vibrations that do not result from the knocking than the cylinder block. For this reason, the knock sensor is less affected by the vibrations that do not result from the knocking when it is provided on the cylinder block than when it is provided on the cylinder head.
Nevertheless, in some cases, the knock sensor cannot be disposed on the cylinder block due to, for example, the layout constraints of the internal combustion engine or the heat resistance performance of the knock sensor.